The supporting structure of a wind turbine at sea usually has a slender design. The supporting structure of an offshore wind turbine for instance typically comprises a turbine tower and a lower portion in the form of a trussed structure, referred to as a jacket. Apart from such a jacket construction, many wind turbines are supported by a so-called monopile. Monopiles are large slender bodies, usually hollow steel or concrete cylinders that may achieve lengths of 50 m and more, diameters of 6 m and more, and weights of 500 to 800 tons and more. Monopiles are huge structures and in fact may be compared to tunnel tube sections. The foundation of a wind turbine of this type comprises a monopile and a transition piece grouted thereon, and forming the connection between the monopile and the turbine tower. It is clear from the above description that a lame part of the monopile extends under water where it takes support onto a resistant substrate in the seabed.
Due to its vast dimensions, a monopile is particularly difficult and expensive to handle. A known method for erecting at sea such a large slender body comprises the use of a jack-up platform on which is provided a large crane able to support the load of the monopile. The monopile is typically transported from the shore to the off-shore platform by closing off its two open ends with closure caps of steel or another suitable material, and letting it float in a substantial horizontal position. The floating monopile (due to the enclosed volume of air) is then dragged to the off-shore platform where it is handled by the crane.
Another known method makes use of a heavy lifting vessel. Such as self-propelling vessel typically has a number of monopiles on board and also uses a crane to upend monopiles, place them in a pile gripper and drive them into the seabed.
The above manipulations of the known method are time-consuming, and, in a typical ease, may take as long as 5 to 7 days to be completed.